Optical phase shifters using polysilicon capacitors embedded in silicon on insulator (SOI) waveguides are known. The charges accumulated on the plates of the capacitors change the effective propagation velocity of infrared light through the corresponding SOI waveguides. To inject and hold the charges on the capacitor plates, an electronic driver is required for each capacitor.
One promising application for the polysilicon phase shifter is an optically transparent switching fabric (N×M matrix) to route bursts of information packets (e.g., internet protocol (IP) packets) or even individual IP packets through an optical communications network. The basic physics behind the matrix is a phased-array architecture which is a proven technique for steering beams of IR (infrared) light in waveguides fabricated in InP (Indium Phosphate) and other materials.
In principal, a multiple-input and multiple-output non-blocking switch fabric can be made through a simple extension of the beam-steering concept. However, obtaining the necessary level of optical performance of a medium-scale switch fabric is non-trivial. This is especially true when the settling time of the switch must be driven down to 10's of nanoseconds as will be required for an optical packet switch. To appreciate the challenges from an electronic control perspective, it is useful to calculate the number of independent high-speed analog signals to control a M×N switch fabric.
The parameters M and N are the scale of the switch where M represents the number of output ports and N represents the number of input ports of the switch. For example, M=N=8 and M=N=16 are typical values. Parameters k and D are governed by specifications of optical cross-talk and optical loss. In general, both k and D will increase as the number of optical outputs (M) increase. D is the resolution of the voltage applied to the waveguides and, thus, corresponds to the resolution in phase accuracy of the phase-shifter. A value of D=8 would be adequate for an 8×8 fabric. The final parameter, k, is the number of signals developed by the multimode interference splitters in the switch and affects the clarity of beam steering and, thus, the overall performance of the switch. K must be 4 or larger for an 8×8 switch. Thus, the number of independent high-speed analog signals is k*M*N=256 signals for an 8×8 switch, and at least 1024 signals for a 16×16 switch. In addition to just the sheer number of interconnects, the ASIC die area, peak switching current, standby power dissipation, and peak power dissipation must be considered.
Typical switching current per interconnection is (750 pF*2.5V/10 ns)˜200 mA. For a 16×16 switch, the peak switching current would be a significant value of 200A. Care must be taken in design of the power distribution network of the switch because transients in the supply voltages and electrical crosstalk can greatly extend the settling time of the analog voltages and, thus, become limiting factors in the switch settling time (e.g., all analog voltages must have settled to within 1 LSB (Least Significant Bit) for the switch to be settled).
For the same 16×16 switch, the die area of the DAC (Digital To Analog Converter) drivers will also be considerable. Depending on the DAC architecture selected, we can expect at least 3 mm2 of die area per DAC to give a total die area of about 3000 mm2, or approximately 5.5 cm×5.5 cm. This area will be subdivided into many smaller driver chips and, thus, the total board area will be much larger when packaging and board-level interconnects are taken into account. The end result is that some DACs will be physically quite far (up to 10 cm) from the array of optical waveguides. Even with careful design, this distance alone can add 2 to 3 nanoseconds to the settling time of the DAC when transmission-line effects are taken into consideration.
Phased-array switches have been produced in InP waveguide materials for RF (radio frequency) applications. The feasibility of using the phased-array concept in an optical communications network has been proven in academic research. Commercial efforts have been made to extend InP-based phased-arrays to create an N×M switch. At least one commercial vendor offers an 8×8 fast packet optical packet switch operating on a different principle, namely, a set of cascaded 2×2 interferometric switches made from Lithium Niobate. These switches are connected in a tree structure and coupled through evanescent coupling.